Ana tiene 15 años. pepita tiene 16 años. julia tiene 17 años. julia es menor que ana y pepita. ana es mayor que pepita y julia. julia es la mayor de las mus. ana es la mayor de las mus.

The answer would be 'Julia es la mayor de las mus.'

The sentences say:

Ana is 15 years old

Pepita is 16 years old

Julia is 17 years old

Julia is younger than Ana y Pepita

Ana is older than Pepita and Julia

Julia is the oldest out of them

Ana is the oldest out of them

Hope this helps :)